MEASUREMENT OF FOREST CANOPY STRUCTURE WITH A LASER PLANE RANGE-FINDING METHOD - DEVELOPMENT OF A MEASUREMENT SYSTEM AND APPLICATIONS TO REAL FORESTS

As the structure of forest canopy is usually complicated in its archit ecture, its 3-dimensional (3-D) structure should be measured to clarif y the effect of the canopy on forest hydrological processes. From a hy drological view point, measurement of canopy structure must be non-des tructive and applicable over a broad area to allow investigation of se asonal fluctuations in hydrological properties across a watershed. in this paper, a laser plane range-finding method was examined as a new n ondestructive way to measure the 3-D structure of forest canopies. in the laser plane range-finding method, the trace of the projected laser plane is detected in the image from a CCD camera and 3-D data are obt ained via triangulation. By scanning the laser plane over the objects, hundreds of scenes of reflected laser traces are recorded. All points on the laser traces can be converted to 3-D data for reconstructing t he objects. We developed an automatic acquisition system, consisting o f a laser unit, a CCD camera, a platform, a video tape recorder and a laptop computer which controlled the system. The precision of this met hod depends on the stepping angle of the laser plane and the resolutio n of the CCD camera. The acquisition system delivers 3-D data with ani sotropic precision to less than 2 cm along a laser plane, and less tha n 10 cm at right angles to the laser plane when the height of the fore st canopy being studied is at most 20 m high. For use in a real forest , three applications were proposed and executed with good results: (1) (Run 1) projecting the laser laterally to measure the positions and d iameters of trunks; (2) (Run 2) projecting the laser from below the ca nopy looking upward to measure positions of canopy foliage; and (3) (R un 3) projecting the laser downward from above the canopy to measure t he shape of the upper boundary of the canopy. The experiments were con ducted in a real forest whose vegetation was a crown-closed, mixed sta nd of red pine and, broad-leaf trees. The data-acquisition procedure w as executed automatically at night. The recorded data on videotape and hard disk were processed in the laboratory, calculating the 3-D coord inates of the laser reflecting points by triangulation. Tree trunks we re reconstructed using 3-D data from measurements taken in Run I. The distance of each trunk from the CCD camera was found to agree satisfac torily with distance measured directly with a survey tape. Measurement s of trunk diameters using the two methods were also in good agreement . The canopy was reconstructed using 3-D data from measurements made i n Run 2, from which vertical distribution of biomass was obtained. A b ird's eye view of a digital terrain map were constructed using 3-D dat a of foliage from measurements taken in Run 3. (C) 1998 Elsevier Scien ce B.V. All rights reserved.